The heart includes a number of normal pathways that are responsible for the propagation of electrical signals from the upper to lower chambers necessary for performing normal systole and diastole function. The presence of an arrhythmogenic site or accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions, referred to here as tachycardias.
A variety of approaches, including drugs, implantable pacemakers/defibrillators, surgery, and catheter ablation have been proposed to treat tachycardias. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying causes. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. It is important for a physician to accurately steer the catheter to the exact site for ablation. Once at the site, it is important for a physician to control the emission of energy to ablate the tissue within the heart.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols that have been proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. Radiofrequency catheter ablation is generally performed after conducting an initial mapping study where the locations of the arrhythmogenic site and/or accessory pathway are determined. After a mapping study, an ablation catheter is usually introduced to the target heart chamber and is manipulated so that the ablations tip electrode lies exactly at the target tissue site. Radiofrequency energy or other suitable energy is then applied through the tip electrode to the cardiac tissue in order to ablate the tissue of the arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signal patterns responsible for the tachycardia may be eliminated. However, in the case of atrial fibrillation (AFib) or atrial flutter, multiple arrhythmogenic sites and/or multiple accessory pathways exist. The conventional catheter with a single "stationary" ablation electrode can not effectively cure the symptoms.
Atrial fibrillation is believed to be the result of the simultaneous occurrence of multiple wavelets of functional re-entry of electrical impulses within the atria, resulting in a condition in which the transmission of electrical activity becomes so disorganized that the atria contracts irregularly. Once considered a benign disorder, AFib now is widely recognized as the cause of significant morbidity and mortality. The most dangerous outcome from AFib is thromboembolism and stroke risk, the latter due to the chaotic contractions of the atria causing blood to pool. This in turn can lead to clot formation and the potential for an embolic stroke. According to data from the American Heart Association, about 75,000 strokes per year are AFib-related.
The tip section of a conventional electrophysiology catheter that is deflectable usually contains one large electrode about 4 to 8 mm in length for ablation purposes. Sometimes, a plurality of long electrodes is used in creating a contiguous linear lesion. The lesion is generally not deep because of potential impedance rise of the tissue in contact with the "stationary" catheter electrode(s) and thereafter the ablation time needs to be cut short. The word "stationary" means that the contact point of the electrode with the tissue is the same point unless the electrode is rollable or rotatable so that the contact point changes from time to time.
In some cases, the contact of a stationary electrode of the conventional catheter with tissues reportedly results in potential tissue adhering to the electrode. A rollable electrode on a stationary catheter is in need to reduce the tissue contact impedance rise and temperature rise by slightly moving the rollable electrode back and forth so that the temperature rise is decreased by the surrounding fluid or by the irrigation fluid.
However, the conventional catheter simulating the "rollable electrode" phenomenon by dragging the catheter back and forth has one big drawback. By moving the tip section of the catheter back and forth, the target tissue site may get lost. Therefore, it is imperative to the keep the catheter stationary while creating a real linear lesion, not a contiguous linear lesion for atrial flutter or atrial fibrillation indications.
Avitall in the U.S. Pat No. 5,242,441, teaches a rotatable tip electrode. Said electrode is secured to a high torque wire for rotation and electrical conductivity. The tissue contact site is always the same spot even the electrode is rotated. Moreover, a movable band electrode has been recently introduced to the market to simulate the "rollable electrode" concept. Since the said band electrode does not roll, the contact surface spot of the said band electrode with tissues is always the same spot. The potential coagulum at the contact electrode surface spot due to impedance and temperature rises, would not go away because of its relatively stationary position of the rotatable tip electrode or the movable band electrode.
While a radiofrequency electrophysiology ablation procedure using an existing catheter has had promising results, the tip section of a known catheter usually has a fixed non-rollable electrode when contacting the tissue for ablation purposes. Therefore there is a need for an improved catheter and methods for making a deeper and larger lesion in the cardiac tissue employing a rollable electrode means on a "stationary" catheter during ablation procedures.